Make the cake: Preheat the oven to 350˚ F and line a baking sheet with parchment paper. Cut off the top one-quarter of the pumpkin; reserve. Scoop out the seeds with a spoon and scrape the inside clean. Place the pumpkin on the prepared baking sheet.
Whisk the flour, cocoa powder, salt and baking powder in a medium bowl. Combine the butter, brown sugar and water in a medium saucepan over medium-high heat. Bring to a simmer, stirring, until the butter melts and the sugar dissolves, 3 minutes. Whisk in the chocolate until melted and smooth. Remove from the heat; whisk in the eggs, vanilla and scotch until smooth, then whisk in the flour mixture until combined.
Pour the batter into the pumpkin, filling it about three-quarters of the way. Add the pumpkin top to the baking sheet, if desired (don’t put it on top of the pumpkin). Bake until the cake is browned and rises slightly out of the opening of the pumpkin, about 1 hour. Reduce the oven temperature to 300˚ F and continue baking until a skewer inserted into the center comes out clean, about 50 more minutes. Let cool 3 to 4 hours before slicing.
Meanwhile, make the butterscotch sauce: Melt the butter in a small saucepan over medium heat. Add the brown sugar and cook, stirring, until melted and bubbling, about 2 minutes. Whisk in the heavy cream and cook, stirring occasionally, until thickened, about 10 minutes. Stir in the pumpkin pie spice, salt, vanilla and scotch. Transfer to a small bowl and let cool slightly.
Slice the cake into wedges, cutting through the pumpkin. Serve with vanilla ice cream and drizzle with the butterscotch sauce.
We used a small sugar pumpkin for this cake. Try to get a little bit of pumpkin with each bite — it offsets the sweetness of the cake and toppings.